They traveled by train. The record of events for Company G (included under “Unit info” in the service records on Fold3) states that the regiment left for Virginia on June 12, 1861 and arrived at Manassas on June 17. Another company states they left on the 13th and arrived on the 18th. The record of events for Company I specifies the movement by rail, which it would have to be to get to Virginia that fast. The record of events for Company B doesn’t talk about how or when they got to Virginia.