I've doubtless said all this before elsewhere, but specifically re your comment about your Sunday ride:
If I’m heading out on a 100k bike ride (was my normal Sunday thing), I’m going to have to work that out as not eating isn’t really realistic.
For a non-stop 100k ride these days I'd knock a couple of units off my morning basal (taking 6 or 7U vs normal 9U), for an 200 (with a stop!) I'd knock it down further (to ~3U). I would probably survive without reducing my basal, but I'd need to eat more in the latter part of the ride for sure - at least 100km is short enough (time-wise) that I could live with that, I think a century or 200 would be pushing my gut's desire to keep eating so much for so long.
For a 100km I don't tend to stop (for proper food/cake/etc), but I do stop on the longer rides to eat proper food part way. If you are stopping I'm sure you'll be fine trying without changing your basal even if you do start to run low on either leg (as you can fix things and eat during the stop).
If I don't stop, I tend to eat ~30g carbs an hour from ~2h onwards (I eat something uncovered when I leave, which tides me over for a couple of hours alongside my liver being helpful), if I do stop and eat something a bit more substantial for lunch (I don't bolus for this at all) I can get away with eating very little on the way back as the uncovered food tends to drive me up into the low teens for a little while - interestingly if I can time it right I don't go too high, but also don't drop away - presumably digestion is matching carb requirements. It's still a work in progress!
I did once try taking some bolus to cover lunch (1/3 normal in this case) on a century ride, I still went high as I was hanging around, and then I needed to eat much more than 60g of carbs an hour on the way back all the while bumping along just above being hypo. I don't see a useful way to take any bolus - I happily burn off my lunch glucose and still usually need to eat eventually even just running on reduced basal, so short stops it is for the time being!
When I first started riding and was upping my distance, I was still taking full basal and my experience of riding ~80km (without a stop as this was during lockdown, aside from me trying to recover from the hills that is), was that I would need to eat incessantly (well that's a bit of an exaggeration, but I'd probably need at least >60g/h - which I now realise is around the "recommended" rate), which meant taking loads of food with me. This was in the early days of my cycling career, so I hadn't got a very good (high density) set of food stuffs to take with me (so it took up lots of space); I wasn't used to eating while exercising for that length of time (so the combo of eating + exercise would eventually make me feel fairly nauseous); and I wasn't as fit/efficient in general. Nowadays I'd not be too bothered about riding 80km with full basal (especially when I'm riding to work so I'm stopping half way and can fix BG if needed) - so you may find you don't suffer too badly as you've obviously been cycling for some time.
In the early days I'd also run low overnight after a ride like that. Once I realised that reducing basal was an option and started doing it, I'd taper my basal back up to normal over a few days to avoid going low overnight. I don't get this effect very much these days (and I now always take my full evening dose after a ride) unless it's a century or longer, and then I just have a bedtime snack and it's flat all night and my dawn phenomenon has vanished the next day (yay, though perhaps a quite extreme method of achieving this result!
🙂)
If it's a long ride I will eat something uncovered as I leave - If it's leaving first thing in the morning then it's anyone's guess whether the (reduced) bolus I take for my dawn phenomenon will adequately cover that or not (usually not, though my last long ride was much improved, and my dawn phenomenon seems to be abating somewhat in general, so the jury is still out). If leaving a bit later in the day (without any/much active bolus IoB left over from breakfast) then I'd probably have 20-30g uncovered as I head out the door depending on BG level.
Sorry for all the repetition.
I trust we'll get a report as to your progress on Sunday evening
🙂