The new Magellan Switch Up has the ablity to add an extra battery - not sure what time that extends to but dcrainmaker has a review. Not Garmin but functionality is very similar.
been running a year with the Sheffield 1/2 in 1:38 under my belt - about to extend my long runs over 20k ready for the VLM next year (ha, right!) - been doing recovery runs of 7-10k - any thoughts on whether I should extend these too?