OTOH land-based trips are pretty hard from TIL.
Yes and no. Kind of depends on how you define "hard."
There are only two scheduled trips a week to the mainland: Saturdays and Wednesdays, but Wednesdays can be iffy.
Saturday is the "regular" transfer day when most guests are leaving or arriving. If there are no guests scheduled to leave or arrive on Wednesday and if supplies aren't otherwise needed, then there's no trip to the mainland that day.
We arrived on a Saturday and stayed for two weeks. I wanted to take a trip back to the mainland to check out the Belize Zoo. As it so happened, there were two guests scheduled to depart and two more to arrive on the first Wednesday we were there, so that's the day I went to the zoo. I didn't know for sure that would be the day I was going until about a week or two ahead of time. Otherwise, I would have gone on the Saturday in the middle of our stay, but I wanted to and did spend that day getting my nitrox certification.
So the bottom line is this: if you're planning to stay a week at TIR, which is probably the amount of time most guests spend there, and if you plan to arrive on Saturday, you may or may not be able to do a land trip on Wednesday via the TIR boat and won't necessarily know ahead of time.
You could charter a boat or a helicopter but obviously that would be an added expense. The TIR boat is included in the price of admission but if you do this then you have to be back at the dock by 2PM, cutting into the amount of time you would spend on the mainland.
I suppose another way to do it would be to stay a Wednesday to a Wednesday and plan a mainland trip on the middle Saturday. But that would be four one-hour twenty minute boat rides in a seven days plus the Blue Hole trip. Not my idea of how I want to spend my summer vacation as that's too much time on a boat vs under a boat but as always ymmv.
If we had been staying only a week, I would have skipped the zoo trip in favor of just hanging on the island. To me, it's not so much a matter of "hard" as it is a question of whether the time and expense of a mainland trip would be worth it when balanced against time spent on the island relaxing and diving.
Note: Peter, I was composing the above while you were posting your "fair enough" comment, so I didn't see it until after I posted. I didn't mean to sound like I was piling on so please don't take it in that spirit. Your comment about adding 3+ hours to the day is spot on, btw.