I'm training for a century in August, and have been a ovo-lacto-pesce-vegetarian (I eat eggs, dairy, and occasionally fish, but no mammals, birds or reptiles) for over 15 years.
I rode 150 miles in two days last summer (my first event ever) for the Arthritis Foundation.
I found it easy enough to keep fueled without meat, but since I've been doing it for so long it wasn't anything new to me. I don't eat a ton of soy (tofu or tempeh) products but I do enjoy them.
If you're considering changing to a plant-based diet I think you should be able to do this without sacrificing your training. It is, however, easy to gain weight when learning how to eat vegetarian because swapping lean protein for things like cheese is an easy way to fuel, but not the healthiest way to go. Explore different beans, lentils, quinoa, and find what appeals to you, taste and texture.
Finding healthy, tasty alternatives to meat does take a little trial and error, as there are lots of meat substitutes out there that are delicious and many that are wretched, some that work with one's digestive system and others that can be, well...not so pleasant.
For fuel during rides I love the oatcakes from the Super Natural Every Day cookbook by Heidi Swanson. They're full of oats, nuts, and coconut oil so they fill me up and give me energy while riding, but are also small enough to pack easily.
I'd suggest getting some vegetarian cookbooks from the library and leafing through them to find recipes that sound good to you, then try a few out before committing the cash to books. Once you get the hang of it you can take your old favorite recipes and adapt them to be vegetarian.
Good luck with whatever you try, and best wishes for your century!
S