Hi Dave
Sorry to hear you had such a difficult time with your recent expeditions. You’ve discovered the hard way that even the “easy” GM hills are often far from trivial… adding the time constraints of after-work activations make for a very challenging combination! Please don’t get discouraged.
Key to success lies in thorough preparation of every aspect, although it’s only a “quick nip up a hill” a small unforeseen issue can upset things entirely so parking, route and timings are all potentially critical. There are many sources for the first two and for timings I can recommend developing an itinerary system that accounts for all the elements that make up the expedition including “Faff” time - travel, parking, kitting up, ascent, setting up, operating, dismantling, descending - to be sure that what you’re attempting is possible within the available timeslot.
Regarding ascent times, look carefully at the route you intend to follow, work out the distance and total vertical ascent and apply your own version of Naismith’s rule - you can make assumptions at first and as you gain experience you can modify it to generate an algorithm that suits your own ability. And with a bit of luck as you do more and more hills tweak it to allow for your increasing level of fitness 
HTH and isn’t just a lesson in egg-sucking… good luck with your activating and look forward to hearing of your further exploits!
73 de Paul G4MD