Aye, it's a tough one. Sometimes having people to chase can push you on, but also they can encourage you to push harder than you should and you pay for it. I've found that a lot of this is experience as once you've done a few hour-long climbs you get a feel for how hard you can sustain things over the hour (obviously if you have a power meter then you can use that). As a general rule of thumb though, the first half should be as comfortable as possible. Most of all though, enjoy it. Riding in the mountains is a beautiful thing and you're bound to have some great scenery to admire.