Firstly, I was just about to suggest that the seat is too low but you've changed that now. I'm pretty sure you will feel much better tonight on the way home, pedalling will feel much more effortless.
I think you have bitten off a bit more than you can chew at the moment but that's fine, I like your enthusiasm and confidence. Just take the journey nice and slow until you build up your heart and lungs and your muscles.
Nothing wrong with stopping now and then for a breather, its always a good idea to have a handful or two of some nuts and a few mouthfuls of whatever drink you carry with you (you do have a drink bottle right?). Cereal bars are also a nice snack to have when you stop.
And use the appropriate gears, if your tackling a bit of an incline then drop down to a nice and easy gear and take your time.
Also, I don't know what you ride but if your bike is a mountain bike with knobbly tires then change them to a much slimmer profile and slicker surface, this will really make a difference as it lowers rolling resistance.
I would have said to start with do your journey every other day so you give your body time to recover, at least for the first two weeks or so.
And remember, the body is like an engine, no fuel, no power. Make sure you are eating lots of carbs i.e. pasta/potatoes and make sure you are eating sufficient protein to rebuild your muscles.
I hope this helps.
EDIT: Beaten to it ^ Fastest finger first