It sounds like you are not used to road bike gearing yet. I too ride a mountain bike (with slicks) on group rides sometimes and I always feel really strong on it. Something with the smaller gears makes you have higher cadence so you have more torque most of the time. You can respond faster to surges etc. On the road bike the gears are bigger so you may find you want to shift to easier gears for a while, and judging from what you are saying about the knee pain it sounds like you are trying to push too big of a gear. It's a lot better for your body to do a higher cadence with a smaller gear than 'mash' out a big gear. I'm sure it's going to take a while to get used to the roadbike. You two are still strangers at this point, it's going to take at least a hundred miles or so to get a true feel for it. Congratulations for venturing to the 'dark side' (i.e. the roadie world)