I swear I feel like once you become a triathlete, you can never go back in terms of training philosophy. It's why it kills me when I see these HS athletes who are top junior triathletes and run sub 15 5k off tri training, then get forced to D1 running programs, and coach axes out the cross training, the very thing that likely helped and allowed said athlete to get to the top end results.
How often for 6+ years would she do a long hard interval run and then recover by swimming or biking and not running again for 24 hours.....whereas what's the recovery for a runner???? Oh an "easy" 8k or easy 30 mins, etc.....when the answer to your question is, how do runners recover? By running....that's a very hard concept to follow for an elite athlete when they haven't been prescribed to that philosophy in many many years.
Now with that being said, there is sport specificity and with the marathon, you can't fake running 26.2 miles. You maybe can fake 18 or 20 of those 26.2 miles, but that last 10k is going to kill you if you don't have some volume on the legs. But I think there are some "junk" miles that an marathoner can subsitute with a bike to keep their body on schedule.
ETA: But there is also an thought that I likely wouldn't really want an itu triathlete to turn to marathon for their next immediate sport of choice, simply because it's f'ing hard to train up and race well at. As I said I think you can fake 70% of the race on fitness, but then you can hurt badly. So I likely wouldn't have prescribed an athlete like GJ to go to this specific distance, if she only wanted to run. But again, I think a lot of this was taking advantage of her gold medal and the "dream" that sponsors see/hope/think and thus $$ to train and live as she wanted (as a family in the U.S.), and that was a lot of variables that made it hard to turn down. So in that aspects I would think this is an unique case whereas most triathletes would make this type of change.
-USAT L2 coach, M.S. Exercise Physiology https://www.instagram.com/alloutmultisport http://www.aomultisport.com