The terrain isn’t difficult - straightforward hiking with no special gear required - but you can make it as easy or hard as you like by varying the number of days you do it over. Famously the UTMB is a race that does roughly the same route in one push over two days, which is definitely difficult!

The trail I followed for the TMB was not difficult, though difficulty depends on the trail you choose plus how much you want to cover in a day. I had good hiking boots, poles, rain gear, and a not-too-big pack with water, food, and clothing. I did not camp.