I do a lot of endurance events and am in the same boat as you. Unfortunately for what you want to do you’ll likely need a dual training/race setup. This is only for now as the technology catches up. This would involve having an Apple Watch for every day use, and almost all training sessions under 6 hours (fully charged device).
1.) you should definitely wait until the new Apple Watch comes out in 10 or so weeks because the battery life may be a non issue at that point (but I doubt it). If you decide to get the 4 anyway it will then be discounted.
2.) The Apple Watch will be fine for your training sessions that are under 6 hours (for the Apple Watch Series 4, again- wait).
3.) For events (races and long training sessions) you should look into getting something like a used Garmin Fenix 3 which can be had for maybe 50 dollars. I have one and use it for adventure travel, long distance hiking (30+ miles/day), long distance biking, etc… The tracking can be set to ‘economy mode’ and it makes the battery last days with periodic charging. Note this option requires an external heart rate monitor but you’ll have all of your metrics, albeit on another platform. You could also look for a cheap Fenix 3 HR. I think the key is to minimize the times you actually need to use it so you can use your Apple Watch as your main device and not have to manage so much external data.
4.) look at getting a sapphire watch face vs the sport model, since it dramatically improves the screen durability (within reason). You’ll be amazed how many times you bash your wrist into branches and rocks on a long trail run until you start keeping track. This won’t matter cost-wise too much if you consider adding the cellular model which somehow doesn’t hurt the battery life too much.
Again this is an issue of having to wait for technology to ‘catch up’. I preordered the first Apple Watch and stopped using it for these types of activities almost immediately. The battery life was unfortunately not up to par, and it was not durable for the environments I needed to bring it into. Now the tech is much better and the benefits are great with the introduction of the cellular options, automatic calling of emergency contacts for detection of things like rapid heart rate/arrhythmia while not moving (I’m a doctor and actually trust these functions), and longer (relatively) battery life : most importantly when you’re doing something like surfing, or are out on a long run without your phone you have recourse if there is a medical emergency. This finally pushed the pluses ahead of the minuses for endurance/adventure athletes. Once the battery life is improved to >24 hours of continuous tracking then we’ll finally be in a place where we can go down to a single device. Durability could also stand to be improved.
I can’t tell you how much it drives me crazy to have things divided into Apple Health and Garmin Connect (the workouts do get logged into Apple Health but they don’t have any details, just workout duration). My ideal setup would be just the Apple Watch, my automatic analog dive watch for ‘nicer social events’ (and diving!). For now it’s those two plus the Garmin for when I know I need the data and/or it’s going to get bashed around.
Good luck and let’s hope the iPhone/Apple Watch event brings us good news!