I had a REALLY bad day not long ago when I knew I needed to go out and really didn't want to. I took inspiration from an episode of Friends...random, but stick with me. Not knowing if you are a fan, or ever watched it, I will explain...there is an episode where Chandler gets freaked out about marrying Monica and to help him get over it and get ready for their wedding (that same day), Ross tells Chandler "We're just going to have a shower!"...and then "we're just going to put our shoes on!"
It helped because I started thinking in the same way...doing the tiniest thing and not thinking past that. I put on my top...then I put on my shorts...then I put on my socks...then I put on my trainers...then I got my headphones out of my bag...then I put my key in my pocket...all the while all I did was concentrate on the one tiny little task to do next (and putting on shoes is a REALLY tiny task!)...by the time I got dressed I thought, I'm just going to go downstairs, then I'm just going to step outside...by the time you get to that point you realise there's nothing stopping you from just starting, even if you think I'm just going to run to the end of the street.
Before you know it you'll be running...because sometimes I think the prospect of going out is ridiculously hard, even if it is only 30 minutes!!
You can do it! The fact that you want to is one step already accomplished!!