If your fitness is in doubt, get a heart rate monitor.
I am not going to tell you what number you should use for maximum beats per minute, perhaps ask your physician?
When you start climbing a hill, frequently look at your monitor, and when the number gets up to where you do not like to see it, stop. I then get off the bike and keep pushing up the hill walking, but at a slower pace so my heart rate drops. Before I even left home, I knew that I would have to go up the hill in the first photo. And months before I got to this hill, at a 13 percent grade I knew that I would walk up that hill.
I do not have heart problems, but I am 69 and do not want to develop any heart problems so I am keeping my heart rate in a reasonable range when I am exerting myself.
I am trying to get in shape right now for a bike tour in April and also a 200k brevet at start of April. I am going to the gym three times a weeks. Besides some strength exercises, I also spend an hour on either the stair-master or on an exercise bike. I do not know about conditions where you are, but where I am, we had snow storm about three days ago, and another scheduled for this coming Thur, so I am doing what I can off of the bike to get ready.
I have never heard of anyone riding through the night to avoid traffic on a bike tour. I want to be as visible as possible to the car and truck drivers. That means high visibility clothing and a flasher on the bike. It is much harder to be that visible at night, a flashing light is attention getting but a driver can't get any depth perception on that and can't tell where you are. Lots of reflective clothing would be needed, and a driver that is half asleep in the middle of the night is not someone I would want coming towards me. Second photo, this was the worst visibility I have ever seen for a bike tour, I pulled over planning to wait for the fog to lift somewhat, but while there I saw that the few cars going past were going so slow that I decided to trust that my two rear flashers would alert drivers to my presence. So, I started riding again and the cars that came up behind me apparently did see my flashers as they slowed down.
On my last tour, I was in Canada (I live in USA), and was where you ride on the right side of the road. I started the trip with reflective stuff centered on my bike but later moved it to the left side of the bike, I think the car drivers started to give me more room that way when the reflective stuff was closer to them. See third photo, this was a rainy day so visibility was poor all day long. Not only does the rain impair visibility, but the car driver with a busted defroster and worn out wipers on the way home from a hard day at work is the one you have to worry about.
More recently, I have also bought a reflective belt with shoulder straps that I can wear over my rain jacket for more visibility, but have not used it yet. If you really want to ride at night, you should get some good reflective stuff. I assume for Audaxing you need reflective material for night riding, check those criteria and consider that a minimum.
When I go on a bike tour, I am there to see the sights, I do not ride in the dark as there are no sights to see.