It would be beyond silly for a company to upgrade a product, then kill it the next month with a new version, so like you say, I wouldn't expect the 5 anytime soon.
As for its being a response to the Surface, I think actually it's a response to the mini. Come Autumn when both are updated it wouldn't surprise me to see the iPad 5 base storage start at 32GB and come in 64GB/128GB options, while keeping the mini at a base storage option of 16GB (and still topping out at 64GB), that way creating the illusion of "a deal" (since now the 128GB option is the same as the 64GB option was in the previous iPad 4) and also it provides the element of "premium" in the full sized iPad, another differentiating factor between it and the mini.
One could look at it differently and see it's actually very predictable - they're both coming at the end of Summer/beg. of Autumn, one year (the "normal" refresh duration) after they were both launched. March rumours were never numerous, credible or logical, and WWDC if that were to happen would be very strange indeed.