Rastas won his first medals at the Finnish national championships in 1917, winning gold in the 10,000 metres and silver at 5000 metres. At the 1920 Summer Olympics in Antwerp he represented Finland in the cross-country race, placing 18th; the race doubled as a team event, which Finland won, but only the results of the best three finishers (Paavo Nurmi, Heikki Liimatainen and Teodor Koskenniemi) counted for the team result.