Running outdoors is very different from the treadmill but is brilliant. The thing to do is to start off slowly and not get too out of breath - be able to speak a good few words, not be gasping for air.
The ground stays under your feet, you have to propel yourself along, unlike the TM and it can be uneven which means you have to watch where you are going. It is possible/probable that you might find your first run outside a bit harder than you expect but don't worry or let it put you off, you just need to get used to the transition.
For Parkrun the usual suggestion is to do it as part of the plan, so if Saturday coincides with w7r3 (or whatever) start off with the warm up walk, run the 25(?) mins and walk or walk/ jog the rest.
I'm sure running outdoors and PR will become your favourite things.
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