Taking pains to enjoy oneself at great speed, covering in this way first of several miles (9)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A charade of MASOCHI, an envelope (‘covering’) of SO (‘in this way’) in MACH I (Mach 1, the speed of sound, ‘great speed’); plus S (‘first of S everal’) plus M (‘miles’).