Sometimes you may be driven indoors for your exercise due to weather or darkness. The belts on most treadmills are not wide enough for Nordic walking. You may be able to walk at an indoor mall. Some malls open early for mall walking, or you could just go during slow hours. If you work in or otherwise have access to a large building, you may be able to walk the halls. If you have to miss a Nordic walking workout, ride an exercise bike, do some yoga, or do another exercise of your choice so you don't get out of the habit of working out. Get back to Nordic walking as soon as you can. And, don't forget, in winter there's cross-country skiing.