Racewalking technique will let you go faster without running, if you can't walk fast enough for a good walking aerobic workout at your fitness level.
Racewalking has been an Olympic sport for 100 years. It is very technique-dependent and you can keep improving for a long time, as your walking exercise technique improves. Runners who have injured knees or feet may turn to racewalking for a similar workout without the impact, but there's no reason you can't go straight to racewalking, as some people just find it more enjoyable and a greater challenge than fitness walking. Racewalkers are not immune from overuse injuries, but the injury rate is lower than with running.