In addition to the conditioning, it sounds like you are too stretched out. You probably need to address the stem length/angle, and a proper fitting is the only way to get the correct one. The fitting is worth the money, and may also get other potential fitting issues addressed that you may not know about yet.

That was a big jump for you and you stressed yourself. Do push-ups. Get your bike fitted as suggested. You may be too stretched out for now as your mileage is relatively low but you can build up. While many may suggest a shorter stem, I'd say consider moving your saddle forward a tad, perhaps up to a centimeter. That's easy and you may even prefer the pedaling position. Are your bars the right width? Too narrow or too wide? See the pro.