Don't quote me, but I think the rule is 5 minutes per 1 month of age....which always seemed quite short to me, and that may only be for leash walks. We have been taking Dutch for off leash hikes since he was about 16 weeks old, and they were usually good 1.5 to 2 hour hikes, but he had lots of time to stop, rest, swim, play etc. on our way.
I would think if you take it easy on him, you may be fine in a couple of weeks, but maybe someone who has more experience would think differently from me.

Happy hiking!