Hi Armorel
I use a Garmin Forerunner 101. It uses 2 x AAA batteries. They last about 12 -14 hours if using the best Duracells. I switch off and change batteries at a suitable checkpoint on long events, no problem. Switch back on again and it takes up where it left off. Downsides : it is fairly bulky for a wrist device and you need plenty of batteries for a 100 miler. Don't know if they're still available as I've had mine 4-5 years now. Use cheapo batteries and the time can be as short as 4-6 hours.

Hi Armorel.
I have a similar problem and have modified my GPS to reach about 16 hours by adding additional batteries inside. OK for a 50 I suppose. You could make up a battery pack to charge your existing GPS, but you would have a lead from your pack to the plug on your GPS at your wrist. I think that your only solution is one of those larger GPS's with replaceable batteries, and keep spares in your pack. You would need to check whether the new GPS held the data you needed while you changed the batteries though.
John.

I use a Garmin Forerunner 201 for measuring distance walked and speed - but it only has a battery life of about 11 hours, so is no good for something like the 100. Can anyone recommend a device that would do the same job but last for 36 hours or so on one charge? I'm not looking for any fancy functions apart from distance covered.