I'm not sure what zone you are in, so it's kinda hard to answer this question. I do know that the new book does a 1:1:1 ratio of vermiculite, peat moss, and compost which is about what you would have if you didn't add the sand. I really doubt the sand is going to cause too much of a problem, but if you are really concerned I guess you could try adding some more compost to the bin, or even more peat moss or vermiculite to help with the water retention. However, remember that people have been gardening for thousands of years in the soil that they happen to have, and have had success, so I really wouldn't worry about it.

Don't worry, you don't have too much sand. You just see the sand because of the color difference. Also the market bought compost has perhaps a lot of sand in it. The compost i bought from home depot last month was half sand and rest looked like wood chips. But don't worry. You have a mix which is absolutely well draining mix. Plenty of space and fluff for air. I suppose you will use it to grow using soluble fertilizers? The peat, vermiculite and compost will help retain the moisture and still drain the excess water well. It is perfect.