Originally Posted by chasm54

It's difficult to say without knowing what "pretty tired" means. If it's a question of catching your breath, fine. Bt if after an hour you are pretty much done, then another hour after five minutes is going to be too much. Why not try 45 minutes, rest a while (maybe ride somewhere 45 minutes away for lunch?) then ride 45 minutes back. That shouldn't overtax you and it will give you a basis to experiment further.