DH and I just got back from vacation where we had to do a hike. We wanted to go and see a certain waterfall and there weren't any signs or info to tell us how far we had to walk...it ended up being a hike of an hour. Doesn't sound like too much I know but I've been really ill for months now so it felt like a huge trek to me and I was a very active, fit person before falling pregnant.
The main thing to watch out for is over-heating. Being pregnant you're going to over-heat faster than anybody else and that can make you feel very sick. I got over-heated a couple of times on vacation just from walking around the tourist shops and it's a nasty feeling. DH said I went white as a sheet even though I was burning up.
Your center of gravity is going to be off so be VERY careful on the steep climbs. It's so easy to fall over now that we're getting bigger through the belly (I'm 28 weeks as well). I had to climb up these really rocky wet steps and I was paranoid about falling even though DH was helping me and had a strong hold on me. My belly hasn't even gotten that big but it's enough to put me off balance sometimes.
That was my experience from this week. Other than saying the normal things like take lots of water and sunscreen the decision will be yours. Even though you've done this before chances are that you may find it more difficult and uncomfortable now. The heat will probably the biggest factor.
If you do decide to go have fun and be careful. I'm sure your friends would take good care of you anyway.