Cross training for the injured runner

There's been some good discussion herein and elsewhere about GPP vs. SPP (general physical preparation vs. specific physical preparation) for endurance training.

The hardcore GPP believers feel that one can train for a marathon, triathlon, or even an ultra by doing little more than GPP exercises Crossfit, cross training, metabolic conditioning weight training, etc.

My question is this. Can a runner who is already in ultra aerobic condition, but injured to the point that 50+ mile weeks are out of the question, train for and successfully complete an ultradistance race (50-100 miles) on a combination of GPP and sport specific training?

How about a plan like this:

• M-F: GPP training (cycling, rowing, metabolic conditioning, swimming, crossfit, sprinting, hill repeats, Tabata intervals, etc.)
• Sat-Sun: SPP (LSD, running on forgiving surfaces, long slow trail runs, etc.)

Total weekly mileage would be around 30-40 miles, all from two or three runs.

Is it possible?