The transition from little to moderate activity yields the greatest health benefit. Around 6,000 to 7,000 steps per day already captures the largest part of the benefit compared to inactivity. Sitting a lot is particularly risky in combination with little physical activity; sufficient exercise largely compensates for that.
Sitting a lot is in itself less of a problem than it appears: the increased risk of death that studies find in people who sit all day applies almost exclusively to those who also move very little. People who are sufficiently active largely or fully offset that sitting behaviour. The threshold at which sitting becomes risky lies somewhere around 6 to 8 hours per day, but that figure is based on self-reports and broad averages, so treat it as a guideline rather than a hard line.
The biggest step you can take is moving from very little activity to moderate activity. Several large analyses indicate that around 6,000 to 7,000 steps per day you already capture the largest part of the benefit compared with someone who takes only 2,000 steps, which amounts to a substantial difference in premature mortality in those studies. Whether the benefit then levels off or continues linearly up to higher step counts has not yet been fully established, but for most people the first goal is simply this: build consistency.
In practical terms this means: if you currently move little, every step towards a daily routine of moderate walking, cycling or similar activity is the most rewarding investment you can make. The evidence for this is consistent across multiple analyses and applies to adults in general. The association with the outcome is large enough that causality is plausible, even though observational research can never prove that beyond all doubt. A step counter or watch can help you see where you currently stand and whether you are moving in the right direction.
Overview across multiple factors (2 research records, 6 sources). The strength of evidence differs by component; read the answer for the nuance.