So, if you go here: http://history.house.gov/Institution/Session-Dates/Session-Dates/ and compare the beginning and adjournment dates between the early years and later years, you'll notice that congressional sessions in modern times are almost year-round, while it's quite typical for Congress to only convene from December to mid-spring or so in the pre-Civil War period. The number of legislative days (actually in session) increases somewhat, but not by nearly as much as the calendar length of the session.

Essentially, representatives spent a few months of the year living in DC and doing Congress things, and the rest of the year in their home state, which they used (if they were doing their jobs properly) to figure out what people wanted. If things changed at home while they were in session they could communicate by mail. Mail was slow, of course, but the flip side of slower communication and travel is that the represented are also receiving new information much more slowly, and thus public opinion didn't change as frequently.