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Also, there are weekly group rides out of most every bike shop in town - try a few out and put some miles on! There are also several local bike clubs that you can join and thereby have regular rides to head out on. That's probably the safest way of heading out for a long ride...safety in numbers.
Grab a copy of Twin Cities Sports magazine (the uptown YWCA always has copies in their lobby) - pick a few cycling events and have fun! I'm doing the century on the same date as you and (if I'm not working) hope to do the Tour de Tonka..the 65 mile option. I'd encourage you to do it. The timing of it would be a great marker for you to know where your training is at. The information you'd gain from that event would help you to better prepare for the 6th.
Erik's Bike Shop carries a few different books on Minnesota bike trails - and all reputable bike shops have a copy of the latest Twin Cities bike trail map, which you should undoubtedly have a copy of. With that map, you can easily plan new training routes with ease. It's a great way of discovering new trails and seeing different parts of town.
You're in one of the most cycling-loving communities in the country! Your options are endless.
See you on the road!
"There is nothing more frightening than ignorance in action." -X