You weekly distance is excellent. Work and life get in the way of me exceeding 300 regularly, I sit at 250ish.
If you have not already you should try rowing. When you get some speed on water it feels really good.
Not a professional rower, just looked into form when I was using a rowing machine.
I'm thinking of this: https://en.wikipedia.org/wiki/Rowing_(sport)#/media/File:Avi... and if you do little legwork you're doing it wrong.
It's always a great idea to do regular cycling no matter the elevation as it's a great "compacting machine" for digestion
I actually live in a fairly hilly area, per 100 km I would expect to do at least 1,000 m elevation
That leads into riding defensively. I ride with the mindset that everyone is trying to kill both me and themselves. I expect people to pull out in front of me, I expect doors to open etc.
To get a bit more used to being around cars I would recommend getting up early.
But really the main thing is confidence. You have just as much a right to be there as anyone else, and in reality the vast majority of drivers are great. I very rarely have bad experiences with cars. Obviously it takes time to build up confidence, but at the end of the day a confident, assertive rider who signals and manoeuvres well ahead of time is going to be safer than a nervous, skittish rider.
The relative speed between vehicles it's what does matter - keep the flow (or between the waves), the distance and attention (doors, sideways, blind spots, bus stops), be obvious predictable, use your imagination (fly trajectory) and signaling - even take a middle of the road when needed to not confuse someone he can take a chance to overtake you at wrong moment.
For me it’s cycling, I do 300-400 km per week now and I absolutely love everything about it. The feeling you get when you find a smooth bit of road on a still day and you can feel is the wind in your face and all you can hear is the swish, swish, swish of your tyres is unmatched. It feels like you’re flying.