Once again, you are judging Lescott's ability and fitness based on the Premier League/Championship. This is not relevant in the SPL. Davie Weir played until his early 40's, Kolo Toure has just joined Celtic at the age of 35. I can't stress enough the gap in quality, it is huge. Average League 1 players have joined Rangers in the last 18 months and been stand out performers, really superb.
The reality is that Rangers and Celtic dominate games so much, the opposition do not really mount that many attacks per game. Toure and Lescott's of this world are not going to be run ragged in the SPL and IF they are for some reason, then they will be more than capable of dealing with the quality of striker they come up against.