If you hadnt already bought a bike, I would have suggested that you go to a bike shop and get one that's in your budget. Since you already have this hybrid, it totally depends on how soon/fast you want to take it to the next level.
Let me start by saying, if you haven't been cycling, a hilly 21 mile ride will kick you butt!!! It doesn't matter if you're "in shape". Cycling totally uses different muscles and works you body differently than other forms of exercise. I have friends who are in excellent shape, run marathons, and they tire after an 11 mile ride on a road bike because they aren't used to cycling right now. What I'm saying is, don't let the fact that the ride tired you be the reason you upgrade your bike. How long have you had it??
That hybrid is definately heavier than a road bike you may get, but that doesn't make it a "bad" bike. As long as the bike fits you, and you aren't sore because of an incorrect bike fit, it won't injure you. You're in med school, and you work out, so you know the difference between the pain of a good/hard workout, and that of an injury.
In fact, riding this heavy beast around a while will make you a strong rider once you can afford a road bike.
My husband had a hybrid that he rode all over the place - he did organized rides on it in the 30-40 mile range, it was perfect for our trip to South Dakota when we rode 50 miles on the Mickelson Trail (not going to do that on a road bike!); a nice bike for what he was doing. When I started training for triathlons and working at a bike shop as a group ride leader, although he had no problem riding far, he couldn't ride fast. He would get so mad because I would just drop him and be gone! When we rode with friends on road bikes, there was no way he could keep up with us on the hybrid. 2 weeks ago I bought him a road bike, and after 8 months of riding that heavy beast around, he kicks a$$ on the new bike! It's so light and responsive, and his legs are so strong from pushing around the other bike, that this is super easy for him.
So, it all goes back to what you want to do. Don't dismiss your hybrid. As long as you're riding and the bike fits you, you'll be fine, get stronger and faster, and be able to climb hills easier. Considering you're already thinking about upgrading, I wouldn't waste your money on a "low end" road bike just to have one. Sounds like you would quickly out grow that, too

I know from experience, and bought 3 bikes in a year as I kept "upgrading" into a higher price range - fortunately, my bikes are all used for different purposes. But seriously, I'd wait it out until you can at least afford $800-$1000 for a bike (or more), to get something decent that you'll LOVE. There are some nice carbons out there in the $1200 range (Felt makes a beautiful bike at that price).
Hope this helps!!!