Heart rate

So my husband and i have started walking 2 miles per day this last week. I am a young adult female 5’2” 121lb and for a couple years now I have been tracking my heart rate with my fit bit. It normally stays around 100 always. Sometimes more sometimes less. I don’t drink much caffeine and if I do it’s a cup of coffee then lots of water. Now that we are walking my heart rate jumps from 140-170 and I just wonder if that’s ok.... we aren’t walking very fast, but not too slow. Any opinions?