Running distance over time. I still can't run 5k without walking about 20% of the way.
And it's not going fast either, but I'm still proud of the progress. The data is keeping me going.
EDIT: Thanks for the support! The legs still don't want to do, what I want them to. But they will learn, who's the boss eventually.
Others have given good advice, for me making sure I had the right shoes and the right gate when I ran was important but also stretching well before hand, there's YouTube videos that'll show you proper stretches for running.
But the biggest thing was sticking with it, for me it took about 2 months of running/walking for my body to adjust to the impact and then I was fine. And don't overdue it, give your legs time to heal between runs.