Good point VTR. I was specifically replying "fitness-wise"... assuming that a physio would make the right call on the injury front. But with the confession that they ran 17 miles so soon after a week off with PF... maybe it's vital to hammer your message home!
Got to back the others up. If you're prepared to risk missing 3:30, then try to get to 20 miles at 3:20 pace and see if you can hang on. I'd be amazed if you ducked under the 3:20 mark. More sensible option is to go for 3:25 pace IMO... or even 3:30 and just enjoy passing people in the closing miles.
A hard lesson learned, testing out your foot with a seventeen-miler!
As for your question... I missed almost all the last 2 weeks of training for my first!marathon. It's a fantastic way to taper - it stops you risking overdoing it!
For a first-timer, honestly, if you've trained pretty well up to now, fitness-wise you'll be fine. I wouldn't even adjust my target... or at least only marginally. No more than a minute or two different.
The last couple of weeks' training is all about just a bit of sharpening up. And for the average first-timer, that is not going to make a ha'peth of difference. It's all about aerobic endurance, and that will barely change between now and the start line.
IMO, you r time will be faster by giving your foot a complete rest than if you try to do a bit of training.
Keith definitely gives a good answer. And I would definitely definitely avoid 21. I think the brief moment of confidence that you would get would soon be replaced with doubts about whether you'd done too-much too-late.
But personally I would be happy to do an 18 mile long run, as long as it's not too fast. I've certainly done that in the past, and may well do it this time round.