Converting and calculating average elapsed time

I'm trying to use Excel to calculate the average elapsed time (in seconds) it takes a student to perform an online exercise delivered via a web-based learning management system. The format in which elapsed time is reported in this sytem is a string such as "1 min 20 secs", "19 mins 01 sec" or "45 secs". (See attached worksheet.)

Which Excel format (e.g. mm:ss) would be most appropriate for performing such a calculation, and could anyone help with a formula to convert these strings into the appropriate format?

Many thanks in advance.