@leslie - there is a great thread here with tips about altitude sickness!
I have done the trek also, and am not a super fit person and had no issues, the length of the treks each day are spaced well and you have lots of breaks so it is very doable for most people who have an average fitness in my opinion. We had in our group people in their 60’s, people who were super fit, and people who definitely did not seem fit at all - and everyone made it (and loved it)!
@micatravels - the set up for the trek meant everyone could go at there own pace. There was 1 main tour leader and several porters. The guide was at the front, porters throughout, someone always stayed with whoever was at the back.
You don’t have to walk at the same pace or together - you just meet at checkpoints for breaks along the way and at the end of each day. There were several times I was walking and could not see others around (which was really cool!). The guides were fantastic at making sure no one was rushed or slowed down.