It can be tough...and I know that feeling of fear when you aren't logging the miles you want. But its important to bring it along slowly, so keep at the physio and the ice and all that you need to help it heal. I'm not sure the nature of your calf strain..but you can work your lungs one a bike and/or in the pool so some cross training may be in order. Especially swimming, if you have a pool nearby, will give you a good cardio work out and allow your calf to heal better.
I know you have firm dates for a half and a full...but try and focus long term; if you keep re-injuring you could be out for much longer. when you get closer to the races you can adjust your goals to reflect your reality and still have really positive runs!
Hang in there Mitchy - this too shall pass.

Honesty, you might want to consider adjusting your race schedule. If you are still feeling pain in your calf, it's not healed. And it's not going to heal until you rest it, and let it heal completely. I know how much it sucks, but is it really worth dragging out for months?
I had a calf strain several years ago, that probably could have been a 6 week injury. It ended up being a six month injury because I was stubborn and kept trying to come back before it was ready. In hindsight if had had just stopped running initially, I would have been up and running sooner.