LEL 2013 / London - Edinburgh - London Audax

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.

Garmin is more for posterity/Strava, really. No way I'm doing 900 miles and not being able to bore people with the link forever more...

Ah, I read that as weeks rest, as in only a week between events.
If you prepare well for the marathon and don't injure yourself you should be in good stead to transfer to the bike. I ran a half with little training and was in physio for months afterwards so be careful of doing dumb shit in the run.
You could use a normal Garmin etrex that takes AA batteries and swap them or you could use an external Garmin battery pack and Edge 800 like I did for the 24hr. Charging this might be an issue but there are alternatives and it depends if you want to carry a charger/how often you will have decent stoppage time to charge.