The Bucks bowed out of the FA Trophy in midweek when they went down 2-0 to National League outfit Solihull Moors.
Both goals came after the hour mark and Wilkin believes his side are running out of steam towards the end of games.
“I thought we did OK for 70 minutes against a full-time side and a very good and efficient side,” said Wilkin. “There were some good moments where we worked hard against the ball and turned it over and probably could have been a little more incisive ourselves at times.
“But a lapse of concentration has gifted them a one-v-one and the lad has taken it well. But again I think it’s evidence that we blow up around the 70-75 minute mark and run out of steam and make poor decisions off the back of it, not managing the game in the way that you need to in those crucial moments.
“We have made inroads into levels of intensity and fitness and need to continue to do so.
“There is effort going in there and the lads do get into training and they do work hard.
“It’s important they work hard away from when we are together and live in the right way and make sure their fitness levels are up there with what is required. We need to improve that. It has improved, but it’s not improving enough.”
Bucks resume their fight for National League North survival on Boxing Day when they host high-flying Chester.