Blood pressure is good sitting down 120/75. but just walking across room it goes to 140/90. i'm not over weight or in bad shape. should i be concern?

Yes, I think so: You're right, your resting blood pressure is fine. But it is unusual to see such a rise with quite minimal activity, especially at your age. Improving your conditioning would likely help...probably daily walking, something not too strenuous. Or you may be salt sensitive & need to really watch that. But first you should see your doctor; this needs evaluation to exclude kidney/adrenal/other problems