I know that the most efficient way to increase FTP is generally 2 x 20min intervals and that total time at the top end of Z4 is a key factor but is there a work period that is too short? IOW, I have a convienient hilly route that requires me to spend about 5min at FTP per climb, will 40 min total time at FTP on these repeats give me the same benefit as 40 min total time using 2 x 20 min intervals? Thanks.