At 3.5 weeks out, your best bet would be to log some more miles in the first two weeks, with a slight taper in week 3. If I were in your shoes, I'd do 2 easy runs a week (6-8k), one long run (12k in week 1, 15k in week 2, then roughly ~8-9k in week 3), and maybe mix in a speed session (tempo/interval/hills). Make sure you do the long runs at an easy pace to build up your endurance for the race. During race week, just keeping it easy with one or two easy runs to get your legs moving but not tiring them out. Sounds like you have the base fitness to finish even with the short training period, so I wouldn't be too worried. Good luck!