The first couple of miles are a good warm up: climbing up and down along the rocky riverbanks gives you a little sweat. Most people do the trail in 5 days speding four nights on the trail. This makes only the third day very hard. The most miles to cover and the actual climb through the pass.
Plowing through the snow on the third day my girlfriend asked "Martin, remember we why I decided to come along on this trail?". The third day is the difficult one!
I think if you are reasonably fit you can do the trail. It gets even easier later in the summer when there is no more snow.
The experience is great and we are still proud we made it to Bennet Lake!