Hi Guys

Just wanted some advice really. How do i get the most out of my commute? I don't do it everyday, most weeks i will do it 2.5 times (getting a lift one way one day) and combine it with playing soccer and doing weights. I try and get out at the weekend on either my MTB or road bike.
Some evenings i make the ride home longer (adding anywhere between 3-10 miles and in the summer i'll happily add more) but i want to get fitter. I'm hoping to do some sportif's in the summer and perhaps some racing so i want to make my commute better quality.
the route is pretty hilly (for southern england at least - honestly, i've ridden in the alps so i know what a mountain is but this route is undulating). At the minute the cold (and wet) are taking a lot out of me but as it warms up i will have the capeability to do a lot more but what should i do?
I was thinking of doing fartlek style rides? Pushing for sections and easing off in others? Perhaps mixing it up with well paced riding at other times? Then probably adding in a long weekend ride (100m+)? I'm sure that just easy pace mileage isn't really doing me much good so i need to mix it up. I'm not aversed to buying a HRM but i don't have one at the minute (because i wouldn't really know what i'd be doing with it).

Any advice, gratefully received,

Cheers, Joel