Hi Vic. There's no simple answer to this because it somewhat depends on you. It's possible that this much intensity this early in your program could lead to injury. However, it's much more likely that you'll be just fine. You can get away with doing this. The key point, however, is this: it isn't likely to advance you down your training path - without a recovery (or all out rest) day between speedwork (which is what a 5k is) and a long run, your body won't adapt as efficiently. So while it hopefully won't hurt you, it probably won't help you meet your goals as quickly either. That said, sometimes I stuff a short race into the same weekend as a long run. I don't do this regularly, but I have done it. I lived.
One more opinion: if you must do both, it's probably ever-so-slightly better to do the 5k on saturday and the long run on sunday. if you "drain your tanks" with a long run and try to do speed work the next day, that's a bigger injury risk. In my opinion.
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