Take it easy for two or three weeks -run if you want to, but don't push speedor distance. Then yes, a schedule of three weeks building and one week drop-down (that is, not no running, but reduced), repeating is good to maintain endurance. For speed, have you got a parkrun nearby? Running that regularly should help.Maybe look for some cross-country and stuff to mix it up over the winter?
Absolutely if you've kept on a loop then you can jump into the training plan later.
The other thing to do is join a running club, see what runs they're offering. Mine for example will be starting marathon training runs in December (long runs on a Sunday, gradually increasing in length, and often e.g. taking the train away from base then running back, for a change from the local routes.
I'm faced with sort of the same thing myself at the moment - spent 15 months building back from almost zero, after four months off for injury, reached my goals (50K, marathon and 50-miler) and now I'm determined not to lose all that endurance, so I'm doing a three-week schedule for my weekend long runs - one moderate (10-15 miles), one long (18-20) and a back-to-back (20+ followed by 10-12 the next day). Also running cross-country races for my club and the odd other race, mostly trail, although I ran a road 10K recently.