Gradually return to your normal training regimen. You'll be slower of course, much slower at first, but you'll come around. I never ran marathons, more of a trackie, and didn't run another road race until 5 mo. post-AVR, then I stayed in the 3-3.5 mile range until a year out. I bumped it up to 10K, 8 mi & 15K over the next few months and returned to track races around 2 yrs out, for 800 m & 1 mile. Y'know where you get your HR right up there. It's almost 26 yrs now. I've since done 340 more races, but only one half-marathon, except in training where I'd do 3-4 of those a year, just because. I did a sprint Triathlon yesterday at age 69. Had I been 70 I would have won my AG, but too damn many younger 60s in it.