How much is enough...?

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Hi, just looking for some thoughts on this...

I just stated logging again today, and I'm going to start running more regularly than I have been (which is barely at all). My question is this:

When I go for a run its usually at 13mins/mile for about 20-30 minutes. I feel like a massive failure for my pace not being faster and though I'm happy with the length of time I spend doing it at that speed is it even worth it? Would I be better off swimming more instead...?

Any thoughts would be welcome :)