Overriding a few CSS styles to enable the float:
.mktoForm {
    width: 960px;
}
.mktoForm style + .mktoFormRow {
    float: left;
}
.mktoForm style + .mktoFormRow + .mktoFormRow {
    clear: right;
}
Restyling forms means a significant CSS investment.  You may want to look at my 
destyled embedded form example because at a certain point trying to cooperate with Marketo's styles while deeply customizing the form is a losing battle.  You're better off just using their base HTML and then creating the look-and-feel from scratch.