Woah, yes that must have been a shock! I too was devastated when I started putting weight on much to my bemusement, and finally counted what I was actually eating - like you I just ate what I wanted when I wanted it in my 20s and into my 30s, and I honestly thought not that much had changed, but what I wasn't admitting to myself was that as well as my metabolism slowing down, my lifestyle changed quite a bit as I got older, in my 20s I was very busy and active, not so much going to the gym but I was constantly on the go, playing sports, staying up all night studying or going out so lots of calories burnt, plus I was very busy at work all day every day etc. so I just didn't have that much time to eat and it wasn't uncommon to skip meals completely, plus I was very poor and frugal a lot of the time so just didn't buy loads of treats, takeaways, meals out etc. All that changed when I settled down with DH, it turned into big meals cooked together (DH sized portions for me too, sometimes I'd nick food off his plate too, like the greedy soul I am!) then a nice 'treat' of alcohol or junk food in front of the telly afterwards, 'treat' takeaways or meals out, 'treat' extravagant breakfasts or lunches at the weekends. 'treat' coffee and cake with a friend or 'treat' wine and crisps in their garden, basically treat central 
. No wonder I put weight on! I honestly took a while to believe how much I was eating even when I wrote it all down, I really thought I was living a much healthier lifestyle than I had been in my 20s (and to be fair knocking the fags and all night drinking on the head was a healthy step, but replacing it with crisps and ice cream not the best !).
I was and remain so surprised how easy it is to eat double what you need in the day, it sounds like you ought to feel sick and stuffed when you say 4000 calories but when it's junk food and alcohol it all goes down so easily you really barely notice. I know on MN you get the martyr brigade who go on about 'how can people possibly overeat, I feel sick if I eat more than a lettuce leaf' making you feel a greedy pig if you eat more than 500 cals a day, but modern food is so palatable, accessible and frankly delicious, plus our lives are stressful and food is such an easy emotional prop, I totally get how and why people do it!
So you have my sympathies, I have managed to turn my diet and weight around but won't pretend it's been easy and I do still have to be pretty strict with myself (and probably will be for the rest of my life), I know from experience that when I relax my vigilance (e.g. on holiday or at Christmas) it doesn't take long for me to slip back into old ways! I know some people favour a detox/'cold turkey' approach of cutting all sugar, alcohol, carbs etc out in one go but personally I found it more sustainable to make one small change at a time, make sure I could stick to it, then make another change. Loosely these were my changes (can't remember in what order exactly but roughly):
-Replaced sugary soft drinks and smoothies/juices (was on a few cans a day of coke and a bottle of smoothie, probably 500 cals just there!) with diet versions or no-added-sugar squash (I know some people hate these but they really help me), then gradually cut down the amount I was drinking altogether, replaced with water or herbal tea or black coffee...
-Reduce alcohol consumption, first simply by reducing down the number of days a week I had any alcohol to 2 (but no limit on how much), then took the amount down to within NHS guidelines (loosely, I do still occasionally overindulge but it would be a few times a year now rather than a few times a week ). Finding soft drinks you genuinely enjoy key to success here (I really like elderflower), plus ways to replace alcohol as a stress reliever/relaxant/emotional support.
-Cut out snacks between meals, particularly evening junk grazing. I started by allowing myself as much junk/treats/sweet stuff as I wanted, but I could only have it immediately after a meal (which in itself immediately reduces the quantity as you are less hungry after meals). I then took that down to just after dinner, then introduced a few days a week 'junk free'. I now have treats maybe 2 or 3 days a week max and (usually) a sensible portion size.
-Fruit and veg - I was barely eating any before, started by insisting I had one portion a day, then increased to one per meal, then increased that to 2 per meal. Now regularly get my 5 a day without really thinking about it as is just habit to always include veg with a meal and have fruit after rather than 'pudding'...
-Portion sizes. I am much smaller and blobbier than DH who is tall and lean, physics dictates that I just cannot eat what he does in a day and not gain weight, his calorie requirements are at least 500 cals a day higher than mine. Plus he doesn't like sweet stuff as much as I do. So I just have to accept that I eat less than he does at meals. I really hate looking at his plate piled sky high and mine looking stingy in comparison, so I often work around it by having a very small lunch (and I don't eat breakfast at all which is another story), so our dinners can be more equal...
-Exercise - a few injuries plus general disinclination/too much busy-ness means joining a gym or playing on a sports team isn't going to work for me now (although I have a horse which gives me a certain amount of exercise riding and doing yard jobs), but I found ways to incorporate exercise into my day without too much impact - lunchtime or after dinner walks are good (helps with the evening munchies too!), or walking or cycling to places you might previously have driven or got the bus. High intensity exercise (things like Shred) are excellent for weight loss and can be done in 10 minute bursts. But again finding something you actually enjoy doing is the Holy Grail.
-Trying to find ways of dealing with emotional/stress/comfort eating, and finding ways to replace the role food, especially junk, played in my coping methods (could go on about this at length).
None of this meant I immediately lost loads of weight, to get back to a healthy weight I also had to go on a fairly strict diet plan for a number of years, but these things immediately made a difference to my health and also made the diet possible/sustainable, if you see what I mean?
Very best of luck to you!