Essentially, the idea for really long rides is to start off easy enough. Without any gadgets, you're looking at "I can talk in complete sentences while doing this." intensity, and that includes uphill bits where people without a power meter routinely go too hard (and pay for it later).
If you go too hard initially, the return home can be a bit miserable, but everyone's been there, it's not such a tragedy and you'll be glad you've done it. I do suggest planning your first very long rides so that the return part of the ride is on mostly flat terrain where you can ride easy and still be reasonably fast.