That's basically me. I averaged 38mi/week (just ran the #s) before 1st marathon due to work/life constraints at the time. Ran high 15s/low16s 5k in HS.
Ran the marathon conservatively and finished in 2:45 (around 1:21:30 at half). Didn't blow up but didn't feel great either.
Four long runs in 12 weeks (16, 16, 14, 20)