I'm with you - had a place for 2020 and finally doing it this year! I really really didn't want to do an October marathon as I hate training in the summer, but hey ho, here we are.
Firstly - you absolutely can do it. It's 16 weeks from now, so that's plenty of time. You've been there before, so you know you can do it.
It sounds like you don't have the right training plan for your lifestyle. Whilst many are 5x per week, that simply doesn't work for some people. Have a look for a different one, and don't be afraid to tweak/tailor it.
The key is the long run. This is the one thing you really do need to do, so work out a way to get this done each week. For most people this is early on a weekend (or non-working day), but in the summer, with lighter evenings, you might be able to get away with evening (if you can get away without doing bedtime one evening). Ideally it would be roughly the same day each week, so your body has a week to recover before the next one. To make sure you avoid injury, build the distance up slowly, ideally with a drop-back week every few weeks. Put them in your diary now, so you know what's coming.
If you're feeling broken already, I'd be willing to bet you are trying to run too fast. 80% of your running should be 'easy' - a pace where you can chat, feel comfortable, and feel like you could go a bit further after you've finished. If you need to go back to run/walking for the first couple of weeks, then do that. It will soon come back.
If you only have time for 3 runs a week, you need to be smart about it. A long run, a medium run at a slightly faster pace, and some form of interval work (fartlek, hills, timed intervals). Ideally some form of cross training would be handy too - some strength/conditioning (can easily be done at home, without any special equipment) and your cross fit class.
The other thing that may really help you is company. Find a local running group - there are bound to be other people there who are training for it. Having a running buddy on those long runs makes ALL the difference.
In terms of on the day - whatever works for you, BUT - test it out beforehand. NOTHING NEW on race day. No new kit, no new gels, no new drinks. You've got 16 weeks to experiment with gels, blocks and bars, and to decide what kind of kit you like to run in.