Welcome to BikeForums.
Have you looked into the TnT financial requirements? I've considered doing this ride in the past but never followed through because I didn't want to be stuck having to pay whatever part I couldn't raise to meet the minimum entry ($2500 I believe).
Keep looking for another bike. You'll be very sorry with a Walmart bike and finding a decent $300 used bike, that may or may not fit you, isn't going to be easy. Is $300 your absolute limit?
Anyway, ask away in the General Cycling forum. A lot more people read posts there and you'll get some good suggestions.
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The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.