I'd have from March 17 to late July to get bike-fit after the marathon. Not that long, but it's certainly not like I'd be hopping straight on the bike after crossing the finish line.
If you plan to do both then I would advise doing some brick training leading up to about two weeks before the marathon. That would be where you cap any training session with a dose of the other discipline. In theory it's totally possible and without the need to qualify you should be able to get ride fit very easily in the intervening period.
Between now and the marathon you should focus on a couple of riding things. One is being comfortable on doing a 200k and getting going the next day. The other is working out what kind of nutrition/food/hydration works well for you. Keeping in your nutritional comfort zone is fundamental to success in a multi-day endurance event. Once you get above 300k in riding, the event becomes less about fitness and more about psychology and keeping the engine ticking over.
If you plan to do both then I would advise doing some brick training leading up to about two weeks before the marathon. That would be where you cap any training session with a dose of the other discipline. In theory it's totally possible and without the need to qualify you should be able to get ride fit very easily in the intervening period.
Between now and the marathon you should focus on a couple of riding things. One is being comfortable on doing a 200k and getting going the next day. The other is working out what kind of nutrition/food/hydration works well for you. Keeping in your nutritional comfort zone is fundamental to success in a multi-day endurance event. Once you get above 300k in riding, the event becomes less about fitness and more about psychology and keeping the engine ticking over.