That's quite an expensive way to buy a battery, but it may have the advantages of being weatherproof and maybe an attachment to handlebars if you need that. It will probably run for about as long as the Adventurer's internal battery, i.e. approximately double the running time.
If you want considerably more power (running time) then I'd suggest something like the following, although you will have to arrange a mounting method, etc.. They also do an 11,000 mAh version, but that's obviously larger and more expensive and sadly the navigation to "similar" components around that site seems very poor. Note the particular model linked below has no charger, I believe it uses a "micro" USB, sadly AFAIK the Adventurer uses "mini" USB, but adapters do exist.http://www.tecknetonline.co.uk/products/TeckNet-iEP386-6600mAh-Dual%252dPort-Universal-Battery-Pack-for-iPod-products-and-various-mobile-devices.html
Also try searching this forum (and the web) for portablepowersupplies