How to make an asynchronous method?

Question

I have a conditional synchronous method (hereinafter "GenString"). I decided to make it asynchronous. I got 2 variations. But I would like to know how to do it right? I'll be grateful for examples.

async Task<string> GenStringAsynV2(long arrayLength)
{
    return await Task.Run(() => GenString(arrayLength));
}

Task<string> GenStringAsynV1(long arrayLength)
{
    return Task.Run(() => GenString(arrayLength));
}
string GenString(long arrayLength)
{
    Stopwatch sw = Stopwatch.StartNew();

    StringBuilder result = new();
    for (long i = 0; i < arrayLength; i++)
    {
        result.Append(i).Append(" + ");
    }

    sw.Stop();
    StringBuilder timeWork = new StringBuilder("GenString: ")
        .Append(sw.Elapsed)
        .Append("; String Length: ")
        .Append(result.Length);
    Debug.WriteLine("End");
    return timeWork.ToString();
}