How important is warm up/cool down on cycle commute?

[warp9]

Hi. For the last 4 months i've got into and have been very much enjoying a 6.5 mile each way commute on my bike. It's not the most demanding, a big hill at the start and end, then pretty flat inbetween as it's along the canal, I push myself a bit, but don't go too mad.

However recently, my knees and ankles have started to ache a bit. Most of the time it goes when I get the joint working and warmed up, but I don't do any stretching or warm up or cool down before/after and am wondering if I should be?

Any advice?

BTW - I'm 39 1/3 and could do with loosing 1.5 stone.