A multicenter randomized controlled trial, with follow-up at 6 and 12 months. Physically inactive participants with chronic heart failure with preserved or mid-range ejection fraction will be randomly assigned to intervention or control arms.

The six-month intervention will consist of an individualized pedometer-based walking program with weekly step goals, monthly face-to-face sessions with the physician, and monthly telephone calls with the research nurse. The intervention will be based on effective behavioral principles (goal setting, self-monitoring, personalized feedback).

The primary outcome is the change in 6-minute walk distance at 6 months. Secondary outcomes include changes in serum biomarkers levels, pulmonary congestion assessed by ultrasound, average daily step count measured by accelerometry, anthropometric measures, symptoms of depression, health-related quality of life, self-efficacy, and MAGGIC Risk Score.

Inclusion Criteria:

1. Diagnosis of heart failure with preserved (HFpEF) or mid-range (HFmrEF) ejection fraction with New York Heart Association (NYHA) class II or III symptoms. The diagnosis requires the following conditions to be fulfilled:

   1. the presence of symptoms and/or signs of heart failure,
   2. left ventricular ejection fraction ≥50% (HFpEF) or 40-49% (HFmrEF),
   3. elevated levels of natriuretic peptides (BNP >35 pg/ml and/or NT-proBNP >125 pg/mL),
   4. objective evidence of other cardiac functional and structural alterations underlying heart failure.

2. Physically inactive, as determined by a question "As a rule, do you do at least half an hour of moderate or vigorous exercise (such as walking or a sport) on five or more days of the week?".

Exclusion Criteria:

1. Signs and symptoms of decompensated heart failure, uncontrolled arrhythmia or effort angina, severe or symptomatic aortic stenosis, persistent hypotension, recent shocks delivered by the automated implantable cardioverter defibrillator.
2. Co-morbid conditions that would affect adherence to trial procedures (e.g. inflammatory arthritis, active malignancy, renal disease requiring dialysis, uncontrolled diabetes, major depression or other significant psychiatric disorders, cognitive impairment, significant hearing or visual impairment).
3. Major surgery planned within the next 12 months.
4. Life expectancy shorter than 12 months.
5. Inability to walk from any reason.
6. Baseline six-minute walking distance >450 meters. Patients covering more than 450 meters in the baseline six-minute walk test (6MWT) are excluded due to a possible ceiling effect.
7. Pregnancy.
8. Failure to perform the 6MWT.