Weak battery. I couldn't even go for a run without my phone dying.
Harder than I thought. On my first try, the phone was completely dead. Wouldn't come on at all. I disconnected the home button, battery, and the three screen connectors and reconnected. On a second try it worked. I had some dots and bars on my screen after replacing the battery, but some posters said to wait and recharge the battery. I'm glad I waited, the lines are gone and there is just the faintest appearance of blocks. I'll live with that.
I have a precision screwdriver set that contained bits that fit better than the ones from iFixit that I recommend. Get separate containers for organizing screws. If you get bars and squares on your screen after reassembly, don't panic. Charge the battery up and wait it out.
Update: I could not be happier with the new battery. I believe it's even better than the original. Thank you, fixit.