Have not been to Nandi Hills recently... others can comment on 'current' conditions.
Generally... if you want to 'warm up' before then Nandi Hills climb, you can park at the Nandi Upachar restaurant --
You could also take a airport volvo to Sadahalli gate (McDonalds, before airport toll gate) as well, and ride from there --
You can take a left there, and take the Sadahalli mani road, via Club Cabana, towards Nandi Hills (look for inside roads to reach Heggadahalli; some very beautiful sections there), or ride towards Nandi Upachar, and take that left.
On the way back, if you again want to take an airport volvo, the stops on the main road can be tricky... airport vovlo drivers are_not expecting anyone to board the bus for the initial sections. So they might_not stop at some of the designated stops. You could try going to the aiport and taking the bus from there; check beforehand if they allow cyclists go to the parking lot / bus area. (otherwise, it will depend on the security officer on duty.) Or ride a bit towards Hebbal, and then the chances of the volvo buses stopping, should be better. :) That was my experience ~2yrs back.