I've cycled for Deliveroo for about 1.5 years now and browsing at Tour de France. They only do 100 miles a day? I've done 80-150 miles a day 5-7 days a week non stop for well over a year. I use a cheap heavy mountain bike because I don't get road bikes, you wanna be stronger then don't make it easy for yourself.
I have no idea if I could go as fast of them because I dunno how fast they go but I'm shocked by how little they do a day.