potential foot hairline fracture, training advice?

The less you do (weightbearing with the foot), the quicker it heals.  So, keeping fitness and minimizing recovery often come into conflict with those of us addicted to sweating.

I'd say that training (at least running, probably biking for the first few weeks depending on how it feels) would indeed be part of the problem is you're trying to minimize recovery time.

Boot/crutches for first 4 weeks if it's severe is a common recommendation, then another 2-6 weeks without running is typical.  Many will skip the boot, most skip the crutches (but those minimize recovery time - all a balance of what you're willing to do).

Fitness during recovery:  swim, aquajog first.  Then add in high cadence spinning (best on a trainer), but only when there is no pain in the foot during OR after.  Then can get back on the road on a bike, but that's pretty late in the recovery.  Then back to running - which needs to be built back up carefully.

That's just one person's view (wife has a SF at the moment, so it's fresh) and does not constitute medical advice!  YMMV

Good luck and speedy recovery.  Injuries happen, but $uck!

Matt