If you're only riding twice a month I would say that just riding more often will do a lot more good than a different bike. I have basically one day a week I can make time for a long ride and I work long days during the week but force myself to get up in the early morning and get in 15-20 miles on my old rigid MTB a few days a week before work. It makes a huge difference for me in terms of improving my speed and endurance on the long weekend rides.