August may not make a "good" starting month. It's pretty late. But whether or not it makes an "acceptable" starting month depends on how long you intend to take and what route you intend to take. If you go fast and/or take a southerly route, it may be acceptable.
At 100 miles a day, you could be finished in a month. In that case, starting in August would be just fine. Very few people, however, manage 100 miles a day for very long, and even fewer of them do it self-supported, and even fewer of them have a lot of fun en route. It may also be difficult to find riding partners who desire this grueling of a schedule. Days are also getting noticeably shorter by August and September, so you'll need to manage the daylight well.
You can post for riding companions right here on this site:http://www.adventurecycling.org/mag/companions.cfm