Just when I think I'm getting this running thing down

Well I thought my body was getting accustomed to my new found love of running, but I found out today I was wrong. Yesterday I worked out for 35 minutes on the elliptical before my run on the treadmill and I felt great! So great I ran for 17 min instead of just 15. I woke up this morning feeling great so I just knew my body had adjusted to the running..... WRONG! I get on the treadmill and do my three minutes of walking to warm-up (I start a 3mph and increase the speed every 30 sec), once I hit the three minute mark I increase to my run speed (5mph). The first couple of minutes feel fine and I start getting excited. Then out of nowhere BAM! I feel like I am going to die! I wanted so badly to stop and just walk. The only thing that kept me going was sheer determination, and the fact that there was this skinny chick running behind me. If she can do it, so can I! j/k Seriously though, I love running. I'm tired of my body telling me I can't do it! So I pushed through the fatigue and did the full 17 minutes, slowed back down to 3mph for 2min to catch my breathe, then increased my walking speed to 3.8mph to finish my cardio. Though it sucks that the run was sooooo much harder than yesterday, it still felt amazing to finish. I will get good enough to run a 5k if its the last thing I do.