Comparing Self-paced Versus Moderate Intensity Physical Activity Recommendation for Midlife Adults

We will randomly assign (blocked on Gender (female, male, other), body mass index (BMI; overweight \[25 \< BMI \< 30\]) or obese (30 \< BMI \

Inclusion criteria: * ages 50-64 * BMI 25-40 * Sedentary or low-active, defined as participating in \< 60 min/week of structured exercise * Has a smart phone that can connect to the internet at least once every few days and is compatible with smartphone app Exclusion criteria: * Medical symptoms/conditions that may make unsupervised exercise unsafe (specific exclusion criteria were developed in consultation with a cardiologist and are listed below): * Past 3-month overnight hospitalization * Past 3-month treatment for heart disease, including bypass or valve surgery, interventional procedure(s) such as angioplasty (stent), or a pacemaker or other implanted device to control your heart rate or rhythm * Past 3-month inability to walk without assistance due to stroke, transient ischemic attack (TIA), or neurologic or spinal injury * Past 6-month angina, chest pain, or chest discomfort either at rest or with exertion/activity * Past 6-month peripheral vascular disease such as claudication which causes pain in the legs when walking briskly * Past 6-month seizure * Past 6-month overnight hospitalization due to respiratory disease * Past 6-month lightheadedness, vertigo, dizziness, or fainting * Heart, lung, abdominal, joint, or orthopedic surgeries in the past 12-months, or pending or scheduled in the next 12 months * Takes insulin and/or medication that affects the secretion of insulin * Exercise-induced asthma that prevents brisk walking * Takes anticonvulsant medication or medication to treat/control seizures * Diagnosed with cystic fibrosis * Bone or joint problems that make you unable to walk for at least 30 minutes * Physician has said not to exercise because of bone or joint problems * Physician has said a hip or knee replacement will be needed in the next 12 months * Anything other medical symptoms/condition that prevents participant from being physical unable to walk without assistance continuously for 30 minutes * Currently taking or planning on taking medications that affect HR, such as beta or calcium channel blockers or digoxin, as HR will be used to determine the moderate intensity range for participants assigned to that condition * Past 12-month overnight psychiatric hospitalization * Currently pregnant or planning to become pregnant in the next year * Currently participating in any exercise or weight-loss research studies * Household member is participating in this study * Does not have a Rhode Island mailing address OR if only Rhode Island mailing address is a PO box * Unable to receive materials in the mail at residential mailing address * Does not plan to live in Rhode Island for the next 12 months * Unable to speak, read, and/or write fluently in English * Smokers will be eligible, but smoking status will be included as a covariate in analyses