I think you'd be fine. The hike in isn't steep, the swim in not long, and, though there are some places you have to scramble, the guides are good about showing the best places to put your hands and feet. I think you can get a good idea of what it's like from our pictures in the 2009 collection here:
http://www.flickr.com/photos/staceyholeman/collections/

We used Pacz and were pleased:
http://pacztours.net/actun-tunichil-muknal/

Have a great trip, whatever you decide!

look at it on youtube

How long is the hike in? Is the terrain fairly flat for that portion?

I think it's about a mile and it's not completely flat but not steep. You do have to ford a river 3 times during the hike, though. We were there in August and the water wasn't deep (photo in the set mentioned above). Happy trails!

I didn't find it too physically challenging. I would consider myself to be in moderate physical shape. Certainly not above average and I had no problem with any part of the excursion. The walk to the cave was mainly on a flat path. I wouldn't call it a hike, it's a walk. As hopefulist mentions, you do have to cross a river 3 times during the hike, but it's not deep, I think the water was just about to my knees, maybe my thighs at the deepest part. Our guide was walking fairly quickly to the cave because he wanted to get there before anyone else arrived. The swim into the cave is very short. My husband can't really swim and he had no problem. The guide had him go around the edges of the cave and sort of push off against wall. There are some smaller areas to climb through and some walking in dark areas. There is one large wall to climb up but they told us exactly where to put your hands and feet.

It's designed for a tourist to be able to do, I don't think you'd have to be overly fit - I know I wasn't!

Thanks for the great info.!

I hiked ATM last year, at age 67. I think I had more trouble than others in my group because I am short and there are some large boulders to climb. I stayed just behind our guide and he showed me exactly where to place hands and feet. I loved the swimming part. It's an amazing experience. Go for it!