I am a marathon runner, and I'd say you definitely have a good base from which to start. your basic pattern of 4 times a week is solid enough unless you are ambitious about your marathon time. You could keep that pattern, but significantly up the distance of your long run, the intensity of your sessions, and the length of your "other run". So, if your "long run" is usually 10 miles, build that up slowly until you are doing 22 miles around 3 weeks before your marathon. If your "other run" is around 4 or 5 miles, make that a bit longer and add some sections in it where you vary the pace. If you are already doing parkrun, maybe run to parkrun, race the 5k, then run home. There are lots of ways of extending your training while keeping the quality high.
The long runs really are key. They should be done slower than your target marathon pace, but not that much slower, say around 15 seconds per mile slower than marathon pace. Build up the mileage slowly, aiming to peak in time to taper down slowly to the marathon, this is why I do my longest three weeks out. These build your endurance base which is vital for the distance.
I swear by track sessions for marathon training, but I'm lucky to have the use of a running track. I do a weekly session of 800m repeats at a set pace, with 200m recoveries, again building up the reps; at the most I'll do 12 of these. That is a session you'll know about!!! A good way to work out the speed you should be doing your reps is to take your predicted marathon time in hours and convert that to minutes. So if you are hoping to do a 4.15 marathon, your 800m reps should be done in 4 minutes 15 seconds.
Good luck! Marathons are great! The training can be grueling, but listen to your body and learn to differentiate between actual tiredness that means you are at risk of illness or injury, and "tiredness" which means your body is building strength and pushing through will extend your base fitness.