Agreed. The problem I have at the moment is that I’m pushing hard at most Parkruns (although not yesterday) as I’d love to get a sub-20, but I’ve signed up for a marathon so I’m running longer distances too. In the last seven days I’ve run 19km, 28km, 11km, two Parkruns and two rest days. I don’t think aiming for both goals at once is realistic. I need to start taking Parkruns easier, maybe just having a shot at a PB once a month, and focus on getting closer to marathon distance. Otherwise you’re right, something will go wrong and I’ll end up doing neither.
Really sensible plan.......a flat out parkrun once a month or thereabouts in your marathon training programme would be ideal, both as an effort session and as a useful fitness benchmark. If you’re following a decent mixed programme (ie with a fair amount of speedwork) you’ll still find your 5k speed improving - the marathon training does wonders for your speed endurance which helps you hang on to a higher pace for longer.