I wouldn't worry about Brock's conditioning personally. Look at Overeem's past fights in MMA as a HEAVYWEIGHT. In 2008 he fought Mark Hunt in Dream 5, then in 2009 he fought James Thompson, 2010 he fought Brett Rogers and Todd Duffee, and in 2011 he fought Werdum. The guy has fought nobody in the heavyweight division.
The public perception on Overeem is wrong (as usual) and he is overrated. I think a lot of it is his physique and the way he looks in the cage personally that has people fooled. Now I'm not saying he is not good, there is a difference between that and overrated because it is human nature to overreact. It's just the way people are. I'm aware of the fact that he has power and with 4 oz gloves anyone can knock anyone out in the heavyweight division.
Bottom line... Brock is a VERY live underdog in this fight. I just checked 5Dimes and they now have the line at Overeem -150 and Lesnar +130. Win, lose, or draw to lay -150 on either fighter in this fight is stupid. Especially on an unproven heavyweight MMA fighter like Overeem. There is just not enough information and experience on Overeem to justify laying that price.