I managed a 2.57 marathon (pb 2.43)at age 40 off (generally) 3 days of running per week. Certainly not ideal. I had a special interest in doing the race but was very, very busy in the 6 months leading in. I was commuting by bike so had that extra fitness and I had a long history of distance running in the bank.
Typical week, tuesday I didn't start work until either 1pm or 3pm. I'd get out straight after getting the kids to school and run. Usually 2-3 hours of rugged xc across steep cow paddocks. Creek crossings, rock scrambles, shin deep mud etc.
Thursday or friday I'd usually have a 20-30 min run which would include a 1600m effort at near top pace plus a few sprints or a couple of flat out 400s. Sometimes I'd get a chance to run both days so would run say 5k in 20mins on the other one.
Sunday morning was a 5-10k road/xc race in the morning before work in the afternoon. With warmup and down 15-20kms. for the day.
5-7 days per week would be bike commutes 2 x 40-50 min rides.
If you have a fair background of kms in the bank then a short term program like this would work ok to reach a conservative goal. I'm sure that the 150-200kms of mtn biking with a heavy pack I was doing also made a huge difference.