Marin Fairfax - are upgrades worth the $$$?
I've got a '06 Marin Fairfax, only upgrades are SPD pedals. I generally use it for daily commuting, 10 - 20 mi, and it works well for that purpose. Lately, however, I've started to ride with friends on longer 30 - 50 mi rides and it's a challenge to keep up. The people I ride generally have road/cyclocross bikes; I'm in excellent physical shape, but it's an constant challenge to maintain pace with them. So, I'm wondering whether there are cost effective upgrades to the Fairfax? Or, considering I paid $650 for it, is it better just to save my money and, eventually, just buy a new bike? (Saving my money is probably a 6 mo or more endeavour, which is why I'm interested in a cost-effective and appreciable upgrade).
Here are the details of the bike (full details on page 6): http://www.marinbikes.com/bicycles_2..._specs_alp.pdf
Shift levers: Shimano EZFire Plus, 24 speed
Derailleur Front: Shimano R443
Derailluer Rear: Shimano Sora
Cassette: 8 speed, 12 - 24
Rims: Alex DC-19, CNC Double Wall, 28 Hole Front, 32 Hole Rear