as already stated, if you can do 50 you can do 100.
you just need to:
– eat and drink regularly
– not go faster than you feel comfortable with
– get some training for the climbs (although it’s not that much climbing)
on your current rides you should have some idea of your average speed. Use a speedo/garmin or whatever that you can stick on your bars and keep an eye on your average speed to make sure you are not going over it
if you have an HRM you can do similar e.g. you can ride all day irrespective of distance at say 75% of max HR but if you try to do 80% of max HR then you will blow up within an hour (the figures may vary, i just made em up but the principal is the same).
For what it’s worth I’ve done London to Paris in less than 24 hours and a circuit of the M25 (245km) in 12 hours both earlier this year. I’ve also climbed Col de Madeleine which has 1500 metres of ascent in one climb. Not trying to willy wave just indicating that it’s possible.
I’m no superstar cyclist, just a 51 year old bloke who loves cycling.
Keep a steady 15mph pace and keep eating and drinking and bob’s yer mother’s brother