I understand what you are saying. No, pushups and situps won't help the particular back muscles for drawing a bow. I ahve seen bruisers of guys who can't draw my 66lbs longbow to fulldraw. Here is what I suggest. I have it and use it quite a bit.

http://www.cabelas.com/cabelas/en/templates/links/link.jsp?id=0004832414603a&type=product&cm Cat=SEARCHFEAT_all&returnPage=search-results1.jsp&Ntt=draw+exercise&Ntk=Product s&sort=all&_D%3AhasJS=+&N=0&_D%3As ort=+&Nty=1&hasJS=true&_DARGS=%2Fcabel as%2Fen%2Fcommon%2Fsearch%2Fsearch-box.jsp.form1&_dyncharset=ISO-8859-1