Have to agree that 5 years is a long time before getting out and riding. So treat yourself as a brand new rider. You've got the bike so get it to fit first of all. Then try 5 mile rides and increase from there.

On the HR. If the doc says 105 and you hit 95 then good for a starter. Only problem is those BB's if you are on them. They will limit your HR so don't put yourself into the "Legs falling apart- can't breath stage" Well not till you get the milage up.

Just get on the bike and ride to get the body working- the legs pedalling and the Lungs still breathing. I have a system that works for me. I go at a pace that means that the lungs are working hard so -- I have to take--- a breath ev---ery few seconds. Then when feeling OK- For a couple of hundred yards- on the flat for you- push harder so---that-- you-- have to-- breath-- more--often. The dashes are the intakes of breath.

And go by that heart monitor to keep yourself under controll.

And whatever you do- Do not give up. It will be hard initially but things will improve. 5 miles this week- 10 in a few weeks and the birthday ride may be next year.