3 months after my triple I did a 40 mile ride. We don't have flat bits round here but if I had known how hilly the ride was going to be- I would have trained for it. 4 years after, I did a tough ride that proved that I did not have a heart problem- what I had was a re-conditioned heart that although not perfect- was far better than the old one.
Two things will limit you on health recovery right now. One is mental as you don't know how hard to push yourself. The other is the beta blockers you will be on that will limit the effort you can put in so that eliminates the mental problem.
Take it gently- build up within your capabilities and plan the century ride for next year. Hills will not be a problem. And be prepared to modify the bike to take strain off the chest bone till it heals fully. That took about 2 years for me
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How long was I in the army? Five foot seven.
Spike Milligan