I bet you were getting an error to the effect "...not enough arguments for the the function", or something like that. Anyway, it's simple:
> set runTimeMin [expr int(fmod($runTime,60))]
> set runTimeHour [expr int($runTime/60)]
> set fullRunTime "$runTimeMin : $runTimeHour"
To get a little fancier, use the format cmd to make the hour and minute variables always be two digits (i.e. - 02:30).
> format "%02s" yourvariable
...where the arg flags "0" is for zero padding and "2" is the width.