No, it's a portmanteau of "Don't burn your bridges" (because you might need to retreat over them) and "Cross that bridge when we come to it" (worry about it later)
But hark! Another track appears beside the river. Surely this will be the easier route? Surely this will not lead to yet another bridge, larger and more difficult to cross than the last? Yes, indeed, I shall take this alternative track, that I might find myself easier bridges to cross.