hiking recommendations suggest 20% of your bodyweight for the weight of your backpack. that suggests that carrying more for exercise should be possible. consider that hiking can mean several hours of walking during the whole day, while an exercise walk is usually more limited and you don't have any other challenging activities like pitching a tent, chopping wood, etc that you might have on a hike.

if you are inexperienced, i'd start smaller and increase the weight over a few weeks as long as you feel comfortable. i think it is also important to choose a good backpack that evenly distributes your weight and doesn't pull on your shoulders.

i have never tried this myself however. i have done a lot of hiking in my youth, but i doubt my backpacks ever reached those 20%. so don't take my word for it, but double check with an expert, doctor, gym trainer, etc.