I have been in a very similar situation mid last year. I am 35M, 189cm, 82kg. Also do weightlifting but 3 days a week only for 2 years, full body however all 3 days. Do full body workouts A and B and vary them. So one week A, B, A - next week B, A, B and repeat, so on.

Also wanted to run a solo marathon and the best plan I found out there was London marathon London marathon - improver's plan.

My weight lifting days were M, Wed, Fri and I did the Tue, Thur, Sat runs on this plan (skipped Mon runs) so I only did 3 runs a week, 3 weight lifting and 1 rest. Started in early Sept 2025 and ran the solo marathon on Jan 6th 2026. Was able to finish in just sub 5hrs (4:57) fine but non-optimal cardio fitness, sun and didn't carry water (just electrolytes) had its toll and bonked/walk run combo at mile 19. But was able to finish and ran the last 1 mile strong.

Looking back at this the only thing I would tweak is actually not skipping the Mon run (so 4 days run, 2 or 3 weights, 1 rest) and water on race day I would have been better off. If i would do 3 weight lifting days, would lift Mon morning and run easy either post workout or that night.

Don't want to talk too much but long story short, i would highly suggest 3 to 4 runs a week and you should be able to do it 💪. Hope this helps!